1. Field of the Invention
The present invention relates to a wavelength division multiplexing (WDM) system and particularly to a channel monitoring apparatus in a WDM system configured for monitoring of channels of a multiplexed optical signal at each channel monitoring point using an arrayed-waveguide grating (AWG).
2. Discussion of Related Art
A method for monitoring channels in a typical WDM system employs a tone frequency. A configuration for implementing this method, as shown in FIG. 1, includes: a plurality of optical transmitter modules 11-1 to 11-N for transmitting optical signal channels, each channel having a unique tone frequency; an optical multiplexer 12 for multiplexing the optical signals received from the plural optical transmitter modules 11-1 to 11-N; a post-amplifier 13 as a first amplifier, an in-line amplifier 14 as a second amplifier, and a pre-amplifier 15 as a third amplifier each for amplifying a multiplexed signal produced by the optical multiplexer 12 before transmission; a post-signal processor 131, an in-line signal processor 141, and a pre-signal processor 151 which are coupled to each amplifier (13, 14, 15), each for processing monitoring signals at monitoring points to monitor each channel; and an optical demultiplexer 16 for demultiplexing a signal amplified and applied by the pre-amplifier 15 which is the third amplifier.
The plural optical transmitter modules 11-1 to 11-N, each includes: an oscillator 111 for oscillating bias current of a laser diode to make a channel have a unique frequency; and a laser diode 112 for producing an optical signal of the channel in response to the frequency generated by the oscillator 111.
In the WDM system having such configuration as illustrated above, operation of monitoring the channels is as follows. First, the oscillators 111 disposed within the multiple optical transmitter modules 11-1 to 11-N make each channel have each unique tone frequency f1 to fn through oscillation and apply each tone frequency to the respective corresponding laser diodes 112.
Each laser diode 112 superimposes a fine electric signal where a channel has a unique frequency according to corresponding bias current so as to produce an optical signal of the channel and applies the optical signal to the multiplexer 12.
Subsequently, the optical multiplexer 12 multiplexes the optical signals from the plural optical transmitter modules 11-1 to 11-N and applies a multiplexed optical signal to the post-amplifier 13 which is the first amplifier. The post-amplifier 13 amplifies the multiplexed optical signal received from the optical multiplexer 12. The amplified optical signal is subsequently transmitted to and amplified by the first and second amplifiers 14 and 15 and then transmitted to the optical demultiplexer 16.
The optical demultiplexer 16, in turn, demultiplexes the amplified signal received from the third amplifier 15 and outputs each demultiplexed signal xcex1 to xcexn to the WDM system.
At this time, the post-signal processor 131, in-line signal processor 141, and pre-signal processor 151, which are respectively connected to the first, second, and third amplifiers 13, 14, and 15, each converts a portion of the multiplexed optical signal produced by the optical multiplexer 12 into an electric signal at each channel monitoring point and performs a signal processing operation for obtaining information about a relevant channel from the converted electric signal.
The signal processing operation for obtaining the information about each channel from each converted electric signal is effected by using a band pass-filter with respect to each channel or using digital signal processing (DSP) technology after converting the electric signal into a digital signal.
However, it is accompanied by complication to implement a circuit for generating the tone frequencies at the multiple optical transmitter modules 11-1 to 11-N and a circuit for the post-signal processor 131, in-line signal processor 141, and pre-signal processor 151 for monitoring of the channels at each channel monitoring point. Furthermore, various tone frequencies for different usage should be disposed at significant frequency spacings between the channels using the multiple optical transmitter modules 11-1 to 11-N. The number of channels having individually generated tone frequencies is therefore limited. The WDM system limits the number of the channels, thus causing restriction on capacity of the WDM system.
As illustrated above, in the conventional WDM system, implement of the circuits for generating the tone frequencies and monitoring the channels is accompanied by complication and, moreover, the tone frequencies should be disposed at significant frequency spacings with respect to each adjacent channel. This causes restriction on the number of the channels. Consequently, the number of the channels used in the WDM system is limited. In addition to the problem of limiting the whole capacity of the system, optical signals of each channel must pass through a complicated optical/electric/optical (O/E/O) conversion procedure while performing the signal processing operations for obtaining the information on each channel even though each optical signal has a wavelength corresponding to one of the WDM channels.
Accordingly, the present invention is directed to a channel monitoring apparatus in a WDM system that substantially obviates one or more of the limitations and disadvantages of the related art.
An objective of the present invention is to provide a channel monitoring apparatus for monitoring each channel of a multiplexed optical signal produced by a multiplexer using an AWG at each channel monitoring point in the WDM system to check existence/non-existence of each channel, optical signal to noise ratios (OSNRs), and flatnesses and stabilities of gains of optical amplifiers with respect to each channel, thereby facilitating operation, administration, and maintenance of the relevant system and simplifying a complicated conversion procedure where the optical signal is converted into an electric signal and then converted into an optical signal again.
Additional features and advantages of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure as illustrated in the written description and claims hereof, as well as the appended drawings.
To achieve these and other advantages, and in accordance with the purpose of the present invention as embodied and broadly described, a channel monitoring apparatus in a WDM system, includes: a plurality of laser diodes for transmitting optical signals having different unique wavelengths used in the WDM system; a multiplexer for multiplexing the optical signals received from the plurality of laser diodes; at least one optical amplifier for amplifying a multiplexed signal received from the multiplexer; a demultiplexer for demultiplexing a signal amplified by the at least one amplifier; and a channel monitoring unit for extracting signals at each channel monitoring point disposed between the amplifiers and then monitoring existence/non-existence of each channel and detecting gains of the amplifiers with respect to each channel using an AWG module. The channel monitoring unit includes: a plurality of tap couplers each for tapping the multiplexed optical signal at a predetermined percent at the each monitoring point; a switch for selecting one of the monitoring points and optically switching an optical signal tapped by a relevant one of the tap couplers; an AWG module for dividing the optical signal switched by the switch by channels of the WDM system; a plurality of optical detectors for converting optical signals received from the AWG module into electric signals; a plurality of trans-impedance circuits for converting the electric signals converted by the optical detectors into voltage values proportional to current values of the signals; a channel multiplexer for selecting one of signals converted by the trans-impedance circuits; a fourth amplifier for amplifying the signal selected by the channel multiplexer; an analog/digital converter for converting the analog signal amplified by the fourth amplifier into a digital signal before applying it to a microprocessor; and a microprocessor for checking existence/non-existence of each channel and computing gains of the amplifiers with respect to each channel.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.